As known in the art, valve actuation in an internal combustion engine controls the production of positive power. During positive power, intake valves may be opened to admit fuel and air into a cylinder for combustion. One or more exhaust valves may be opened to allow combustion gas to escape from the cylinder. Intake, exhaust, and/or auxiliary valves may also be controlled to provide auxiliary valve events, such as (but not limited to) compression-release (CR) engine braking, bleeder engine braking, exhaust gas recirculation (EGR), internal exhaust gas recirculation (IEGR), brake gas recirculation (BGR) as well as so-called variable valve timing (VVT) events such as early exhaust valve opening (EEVO), late intake valve opening (LIVO), etc.
As noted, engine valve actuation also may be used to produce engine braking and exhaust gas recirculation when the engine is not being used to produce positive power. During engine braking, one or more exhaust valves may be selectively opened to convert, at least temporarily, the engine into an air compressor. In doing so, the engine develops retarding horsepower to help slow a vehicle down. This can provide the operator with increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle.
One method of adjusting valve timing and lift, particularly in the context of engine braking, has been to incorporate a lost motion component in a valve train linkage between the valve and a valve actuation motion source. In the context of internal combustion engines, lost motion is a term applied to a class of technical solutions for modifying the valve motion dictated by a valve actuation motion source with a variable length mechanical, hydraulic or other linkage assembly. In a lost motion system the valve actuation motion source may provide the maximum dwell (time) and greatest lift motion needed over a full range of engine operating conditions. A variable length system may then be included in the valve train linkage between the valve to be opened and the valve actuation motion source to subtract or “lose” part or all of the motion imparted from the valve actuation motion source to the valve. This variable length system, or lost motion system may, when expanded fully, transmit all of the available motion to the valve and when contracted fully transmit none or a minimum amount of the available motion to the valve.
An example of such a valve actuation system 100 comprising a lost motion component is shown schematically in FIG. 1. The valve actuation system 100 includes a valve actuation motion source 110 operatively connected to a rocker arm 120. The rocker arm 120 is operatively connected to a lost motion component 130 that, in turn, is operatively connected to one or more engine valve(s) 140 that may comprise one or more exhaust valves, intake valves, or auxiliary valves. The valve actuation motion source 110 is configured to provide opening and closing motions that are applied to the rocker arm 120. The lost motion component 130 may be selectively controlled such that all or a portion of the motion from the valve actuation motion source 110 is transferred or not transferred through the rocker arm 120 to the engine valve(s) 140. The lost motion component 130 may also be adapted to modify the amount and timing of the motion transferred to the engine valve(s) 140 in accordance with operation of a controller 150. As known in the art, valve actuation motion source 110 may comprise any combination of valve train elements, including, but not limited to, one or more: cams, push tubes or pushrods, tappets or their equivalents. As known in the art, the valve actuation motion source 110 may be dedicated to providing exhaust motions, intake motions, auxiliary motions or a combination of exhaust or intake motions together with auxiliary motions.
The controller 150 may comprise any electronic (e.g., a microprocessor, microcontroller, digital signal processor, co-processor or the like or combinations thereof capable of executing stored instructions, or programmable logic arrays or the like, as embodied, for example, in an engine control unit (ECU)) or mechanical device for causing all or a portion of the motion from the valve actuation motion source 110 to be transferred, or not transferred, through the rocker arm 120 to the engine valve(s) 140. For example, the controller 150 may control a switched device (e.g., a solenoid supply valve) to selectively supply hydraulic fluid to the rocker arm 120. Alternatively, or additionally, the controller 150 may be coupled to one or more sensors (not shown) that provide data used by the controller 150 to determine how to control the switched device(s). Engine valve events may be optimized at a plurality of engine operating conditions (e.g., speeds, loads, temperatures, pressures, positional information, etc.) based upon information collected by the controller 150 via such sensors.
Where the lost motion component 130 is hydraulically actuated, the supply of the necessary hydraulic fluid is of critical importance to the successful operation of the valve actuation system 100. This is particularly true of so-called bridge brake systems in which the lost motion component 130 is supported by or deployed within a valve bridge (not shown) and hydraulic fluid for actuating the lost motion component 130 is supplied via the rocker arm 120. In the related U.S. patent application Ser. No. 14/799,813, structures are described for biasing the rocker arm 120 and a valve bridge-based lost motion component 130 into contact with each other, particularly in systems in which the rocker arm 130 is biased into contact with the valve actuation motion source 110, which, as noted above, may include a pushrod-based valve train. As known in the art, pushrod-type engines have valve trains with comparatively large reciprocating mass and it is necessary to maintain contact between the pushrod and valve actuation motion source, e.g., a cam or cam follower. Consequently, the forces required to control the pushrod motion are often higher than can be reasonably provided by systems that bias the rocker arm against the pushrod, i.e., the valve actuation motion source. Alternatively, where the rocker arm is biased toward a lost motion component in a valve bridge, excessive play or lash in the pushrod-to-rocker arm, or pushrod-to-cam follower interface leads to noise, impact loading, etc.
In order to maintain contact between a pushrod and its corresponding valve actuation motion source, it is known to incorporate spring biasing into the pushrod itself, as illustrated in FIG. 2. As shown, a pushrod 202 includes a sliding member 204 in it, and a preloaded spring 206 expanding the assembly outwards. When assembled to the engine, the spring 206 pushes against the rocker arm, biasing it toward the engine valves, and also biases the pushrod 202 toward the valve actuation motion source. A particular disadvantage of such a configuration is that it creates a potentially high force against the engine valves, which may induce valve floating. This tendency to cause valve floating limits the force that can be provided by the bias spring in this arrangement.